A greaseproof paper is defined as “1) a collective designation of a paper having oil resistance. 2) a paper or board which is extremely resistant to permeation of grease or fats.” in JIS P0001 “Paper, board and pulp-Vocabulary”.
Greaseproof papers having oil-resistant have been extensively used as a wrapping material for foods. In particular, a greaseproof paper is used for food containing much oils or fat components such as a chocolate, a pizza and a doughnut, preventing the oil from permeating a wrapping material. If oils and/or fat components contained in a food permeate a wrapping material, the oily components permeate to a surface with which the food is not in contact, resulting in grease stain which may lead to deteriorated appearance and thus a reduced commercial value, blackening of a printed part which makes it difficult to read letters, deterioration in OCR suitability of a bar code or the like, and so on. Furthermore, the oils may be transferred to fingers or cloths, leading to smear. Thus, a greaseproof paper endowed with oil resistance is used in a part with which a food is in contact.
Fluorine compounds, particularly perfluoro compounds have been used as an oilproof agent for endowing a greaseproof paper with oil resistance. However, a processed paper containing a fluorine compound repels even a print ink due to its high oil-repellence, so that it causes a void in a solid printing area in gravure printing.
Furthermore, during combustion, a fluorine compound having a perfluoroalkyl group generates toxic hydrogen fluoride. Moreover, recently, it has been demonstrated that perfluorooctanesulfonates generated through an electropolymerization process for producing a fluorine compound are extensively accumulated in human or animal blood and the environment such as seawater, and that when being heated at 100° C. or higher, a fluorine compound produced by an electropolymerization or telomerization process, irrespective of a producing method, gives a perfluoroalcohol which is highly environmentally accumulative and other low-molecular-weight fluorine compounds are also generally persistent. Therefore, particularly in food-wrapping applications, it is desired to realize high oil resistance without using a fluorine compound. Therefore, a greaseproof paper in which a paper substrate is coated with a non-fluorine oilproof agent in place of a fluorine compound has been proposed.
It is known that a vinyl alcohol polymer (hereinafter, “vinyl alcohol polymer” is sometimes referred to as “PVA”) can be used as a non-fluorine oilproof agent. PVA is a hydrophilic resin and forms a robust film, so that it prevents oil permeation and exhibits good oil resistance. For example, Patent Reference No. 1 has proposed a greaseproof paper coated with a coating material containing a PVA or a combination of a PVA with a cross-linking agent, and Patent Reference No. 2 has proposed a greaseproof paper coated with a coating agent containing starch and/or a PVA and a fatty acid. For both cases, particularly when a hot and oily food is wrapped, that is, under high-temperature conditions, oil resistance is insufficient. In particular, when a low-density base paper with an air permeability resistance of 1000 sec or less is used, this problem is significant. When a PVA content in a coating liquid is increased for improving oil resistance, water vapor permeability is reduced. Furthermore, since a PVA is a water-soluble polymer, water absorbability may be increased. Thus, when a fried food such as tempura and fried chicken, or a steamed bun such as Chinese bun which is hot by steaming is wrapped, or when it is heated by a moisturing apparatus or a microwave, the inside of the package is filled with steam emitted from the food, so that a batter coating of a fry or the like absorbs moisture to be excessively soft, taste is impaired, or the paper surface becomes sticky.
Thus, for improving liquid shielding and water resistance, Patent Reference No. 3 has proposed a composite paper having a layer of a resin composition containing a saponified ethylene-vinyl acetate copolymer and a layered inorganic compound. Its oil resistance under high temperature conditions is, however, insufficient.